1. Field of the Invention
The invention concerns a weighted triggering device of the type in which an input shaft drives rotating weights, transforming the centrifugal force to which they are submitted into an effort which tends to displace a movable element along the axis of the device so as to trigger a control system such as an electrical switch or hydraulic or pneumatic valve as soon as the speed of rotation of the input shaft exceeds a preset level.
2. Description of the Prior Art
Weighted trigger devices of this type are frequently used in speed governors or servomechanisms. Accordingly, the most recent watt governors and jet servomechanisms are of this type.
In order to clarify the problem which has been solved by the present invention, FIG. 1 of the attached drawings provides a longitudinal, schematic, cross-sectional view of a conventional weighted trigger device. In this device, an input shaft 10 drives a rotating plate 12 having circumferentially spaced weights 14 capable of pivoting about axes 16 which are tangent to the peripheral edge of plate 12. The effect is to induce a movable element 18 towards a control system such as an electrical switch 20 in opposition to the force of a spring 22. Movable element 18 is thus submitted to two opposing forces: (I) force F.sub.M exerted by the weights and, (ii) force F.sub.R exerted by the spring.
Even if the resistance dF.sub.R /dx of spring 22, i.e., the variation of force F.sub.R as a function of the displacement x of movable element 18, can be considered as constant and always positive, the same cannot be said for the variation of force F.sub.M as a function of displacement x, i.e., of the resistance dF.sub.M /dx of the weights.
This observation is illustrated by FIG. 2 of the attached drawings. This figure represents the variations in force F.sub.M exerted by weights 14 on movable element 18 as a function of the angle .theta. (see FIG. 1) defined between the radial plane passing through pivot axis 16 of weights 14 and the axis connecting center of gravity G of the active mass of each of the weights to its pivot axis 16. Indeed, it can be seen from this figure that force F.sub.M varies along a curve having the approximate appearance of half a sine wave when angle .theta. varies between 0.degree. and 180.degree.. This force F.sub.M is zero when angle .theta. is zero, i.e., when the center of gravity G is disposed within the radial plane passing through pivot axes 16 and inwardly offset with respect to the latter. Force F.sub.M then grows and reaches its maximum value when angle .theta. is near 115.degree., then decreases beyond 115.degree. until it again reaches a value of zero when angle .theta. is equal to 180.degree., i.e., when the center of gravity G lies in the radial plane passing through axes 16 and outwardly offset with respect to the latter.
Because of the generally limited travel of movable element 18, the range of variation of angle .theta., i.e., the possible displacement of weights 14, only extends over a small part of the curve represented in FIG. 2. The choice of the operating range of weights 14 thus simultaneously determines the average intensity of force F.sub.M and the average resistance dF.sub.M /dx of the weight system. Thus, if the operating range of the weights is selected in such a way that angle .theta. is always less than 100.degree., the resistance of the weights is always positive, whereas it is always negative if the operating range is chosen such that angle .theta. is always greater than 130.degree.. The resistance of the weights may also be initially positive, then negative, if the operating range is such that angle .theta. varies between two limit values of approximately 100.degree. and 130.degree., respectively.
Whatever their application, weighted trigger devices of the type defined above are sensitive to a given threshold speed of rotation of the input shaft for triggering some sort of control system, such as an electrical, hydraulic or pneumatic system. Whatever the system, it is of the binary type, i.e., it defines two stable positions which correspond, for example, to the opening and closing of a switch, valve or any other similar mechanism. Triggering of the system must therefore by unequivocal, particularly when the speed of rotation of the input shaft is made to undergo slight fluctuations about the triggering threshold.
In order to increase the speed of response of the device, most known systems are designed such that the force F.sub.M exerted by weights 14 on movable element 18 is as great as possible within the operating range of the device. Thus, referring again to FIG. 2, the operating range of the weights in known triggering devices is usually chosen such that angle .theta. varies from 90.degree. to 130.degree., as illustrated by cross-hatched area I in FIG. 2. Generally, angle .theta. is very close to 115.degree. when the displacement x of movable element 18 has reached its maximum value, i.e., when system 20 is triggered.
Nevertheless, in spite of apperances, this solution is not satisfactory for solving the problem posed by the unequivocal triggering of control system 20. In order to clarify the reasons for this, we refer to FIG. 3 in the attached drawings, which represents the variations in opposing forces F.sub.R and F.sub.M applied to movable element 18 as a function of the displacement x of said element between the two extreme positions x1 and x2 which it is capable of occupying. In this figure, force F.sub.M is represented for various values (N1, N2, etc.) of the speed of rotation of input shaft 10 when the operating range of weights 14 is as in cross-hatched area I in FIG. 2, as is the case in prior known devices. As illustrated in FIG. 3, the resistance dF.sub.M /dx of weight system 14 is then lower at all points to resistance dF.sub.R /dx of spring 22. A detailed analysis of the operation of a weighted device having these characteristics leads to the following observations.
When shaft 10 is not turning, force F.sub.M is zero and movable element 18 occupies position x1 due to the force F.sub.R exerted by spring 22. The point representing the equilibrium of element 18 is then point A in FIG. 3. When shaft 10 is turning at a speed N which is increasing, curve F.sub.M, representing the force exerted by weights 14 against the force of spring 22, moves upward in FIG. 3, in such a way that the point representing the equilibrium of element 18 moves from point A toward point B. As long as curve F.sub.M remains below point B (corresponding to the value of force F.sub.R when the movable element is at x1), element 18 remains stationary. When force F.sub.M becomes greater than the value of force F.sub.R at point B, movable element 18 moves until a stable equilibrium has been established between the forces exerted by weights 14 and spring 22. This stable equilibrium is determined by the point of intersection between curves F.sub.R and F.sub.M. The equilibrium point of the movable element therefore lies on curve F.sub.R between point B and point C, with the latter corresponding to the value of force F.sub.R when movable element 18 is at x2. Control system 20 is then triggered. When the speed of rotation of shaft 10 continues to rise, force F.sub.M becomes preponderant over force F.sub.R, so that movable element 18 remains at x2. By contrast, if the speed of rotation of shaft 10 undergoes fluctuations when the system's equilibrium point is near point C, or between points B and C, curve F.sub.M representing the force applied by weights 14 to movable element 18 may drop temporarily by a certain value with respect to curve F.sub.R, so that the equilibrium point of movable element 18 moves toward point B on curve F.sub.R. Element 18 then moves away from the control system to be triggered. This has the consequence of causing several state changes in system 20 in a very short period of time and producing uncertainty as to the triggering of the system.
The preceding analysis shows that known weighted trigger devices do not make it possible to obtain a clear and precise triggering of the system to be controlled, but rather an uncertain triggering sensitive to passing variations in the speed of rotation of shaft 10. This type of triggering is obviously not desirable since it leads to both hesitation in control and premature wear of contacts.